An effective vaccine against schistosomiasis remains an unfulfilled goal. A
concept I am investigating is to target immune responses at molecules that
are poorly immunogenic during normal infection, as such antigens might be
the Achilles' heel of the organism. The Schistosoma mansoni cercarial
elastase (SmCE) is one such molecule: antibodies which neutralize this
enzyme might abort the infection at an early stage and my hypothesis is
that such antibodies may be induced if SmCE is administered in an
inactivated form. I aimed in this work to improve the levels of protection
obtained in previous experiments with recombinant elastase. I have
generated a His-tagged recombinant SmCE (His-CE) which is enzymatically
inactive. Immunization of BalbjC mice with His-CE on aluminum hydroxide
induced significant levels of specific anti-SmCE IgG antibodies within two
weeks of the first immunization. The IgG subtype profile (lgGl> IgG2a,
IgG2b) was indicative of a ThljTh2-like immune response skewed towards
Th2. Immunized mice showed at least 40% reduction in the mean worm
burden when compared to the control group of mice in 3 out of 4 mouse
experiments. We have generated an active elastase fused to the mouse
IgG2a-Fc region (CE-Fc), which was found to be immunogenic and
promoted a partly protective anti-elastase IgG response in BalbjC mice. A
plasmid encoding for an inactive CE-Fc was constructed by site-directed
PCR mutagenesis, which is expected to be more immunogenic should used
in future work. In this work, we showed that the native CE cleaves mouse
IgG and human IgGjlgA, and potentially exists in a polymeric form. Finally,
a computer model for SmCE was constructed and used to map computer-
predicted B-cell epitopes for the molecule. The epitopes identified in the
vicinity of the active site can be used to synthesize a multi-epitope DNA-
string vaccine in future work.